1. Field of the Invention
The present invention relates to a single mode optical fiber having low dispersion and low loss in a 1550 nm wavelength baseband, and more particularly, to an optical fiber having a multi-step core structure and a method of fabricating the same.
2. Description of the Related Art
As techniques for super high speed optical transmission and large capacity communications are rapidly being developed of late, loss and dispersion of an optical fiber restrict transmission at super high speeds and with large capacity. In order to overcome the restriction due to optical fiber loss, a single mode fiber of silica uses a 1550 nm wavelength region in which the loss value is the lowest, and uses an optical amplifier which can amplify an optical signal in the 1550 nm wavelength region. Accordingly, the loss no longer restricts the large capacity transmission at super high speeds, and control of the dispersion is being mentioned as a relatively important technical problem.
However, a typical single mode optical fiber, the most widely used at the present time, is designed to have a dispersion value of zero in a 1310 nm wavelength region. Thus, even though loss is low in a 1550 nm wavelength region, the dispersion value is high in that region, thus limiting use of the 1550 nm wavelength.
In general, total dispersion of the single mode fiber is determined by the sum of material dispersion and waveguide dispersion. Here, the material dispersion is determined by the peculiar properties of a material forming an optical fiber, and waveguide dispersion is determined by the structure of the optical fiber.
FIG. 1 is a graph illustrating the relationship between the material dispersion, the waveguide dispersion, and the total dispersion of a conventional 1550 nm single mode fiber. FIG. 1 describes the relationship between the material and waveguide dispersions depending on the wavelength of an optical signal in a single mode fiber having zero dispersion in a wavelength region of about 1550 nm. As shown in FIG. 1, the material dispersion D.sub.M appears because a long wavelength optical signal is quickly propagated and a short wavelength optical signal is slowly propagated as the refractive index varies according to the wavelength of an optical signal, and has a positive dispersion value in a wavelength region of about 1300 nm or higher. The waveguide dispersion D.sub.W has a negative dispersion value as opposed to the material dispersion. As a result, total dispersion D.sub.TOTAL is determined as shown in FIG. 1, and a wavelength where the total dispersion D.sub.TOTAL is "0" is called a zero dispersion wavelength.
Thus, the total dispersion D.sub.TOTAL can be lowered in a wavelength region to be used, by appropriately adjusting the material dispersion D.sub.M and the waveguide dispersion D.sub.W. However, a material of the optical fiber must be changed to control dispersion due to the material. Thus, a method of varying the waveguide dispersion D.sub.W is used to control the total dispersion value of the single mode fiber. Here, the waveguide dispersion D.sub.W can be controlled by adjusting the core diameter of an optical fiber, the distribution of refractive indices of a core and a cladding, and the difference between the refractive indices thereof. In other words, in order to fabricate a single mode fiber having a low dispersion value in the 1550 nm wavelength region having low loss, the refractive index of the core of an optical fiber must be increased, and the diameter of the core must be reduced, as compared to a typical single mode optical fiber for a 1310 nm wavelength.
A profile having a refractive index as shown in FIG. 2A is disclosed in, and incorporated by referrence to, U.S. Pat. No. 4,715,679 to Venkata A. Bhagavatula entitled Low Dispersion, Low-Loss Single Mode Optical Waveguide, and a profile having a refractive index as shown in FIG. 2B is disclosed in, and incorporated by reference to, U.S. Pat. No. 4,516,826 to Un-Chul Paek entitled Single Mode Lightguide Fiber Having A Trapezoidal refractive Index Profile.
In U.S. Pat. No. 4,516,826, in which the distribution of the refractive index of the core is triangular or trapezoidal, as shown in FIG. 2B, a core diameter 2a and a mode field diameter 2Wo are smaller than those of the typical single mode fiber for a 1310 nm wavelength, and a dispersion value is low in the 1550 nm wavelength region by controlling the waveguide dispersion. However, such a structure must accurately control a geometrical structure, since a connection loss (.alpha.=4.3(a/Wo).sup.2), upon connection of optical fibers to each other increases when the diameters of the core and mode field, are small, and since a micro-bending loss of an optical fiber rapidly increases when the core diameter is remarkably diminished to control the waveguide dispersion. Therefore, in U.S. Pat. No. 4,516,826, the distribution of the refractive index of the core is to be trapezoidal in order to reduce the micro-bending loss of an optical fiber.
An accurate process control is required, however, to fabricate an optical fiber having a small core diameter and a triangle refractive index profile of a core. Particularly, when an optical fiber having a triangular profile is manufactured by modified chemical vapor deposition (MCVD), a center dip phenomenon where a refractive index dips at a core center occurs. Thus, a more accurate process control is required to fabricate an optical fiber having a desired core diameter and a desired core refractive index, making it difficult to reproduce the process.
Other known single-mode optical fibers and their fabrication process are contemplated by U.S. Pat. No. 4,106,850 to E. Marcatili entitled Optical Fiber With Graded Index Core And Pure Silica Cladding; U.S. Pat. No. 4,306,767 to M. Kawachi et al. entitled Singel-Mode Optical Fiber; U.S. Pat. No. 4,435,040 to L. Cohen et al. entitled Double-clad Optical Fiberguide; U.S. Pat. No. 4,822,399 to H. Kanamori et al. entitled Glass Preform For Dispersion Shied Single Mode Optical Fiber And Method For The Production Of The Same; U.S. Pat. No. 5,361,319 to A. Antos et al. entitled Dispersion Compensating Devices And Systems; U.S. Pat. No. 5,559,921 to Y. Terasawa entitled Single Mode Optical Fiber; U.S. Pat. No. 5,613,027 to V. Bhagavatula entitled Dispersion Shifted Optical Waveguide Fiber; U.S. Pat. No. 5,673,354 to Y. Akasaka et al. entitled Dispersion Compensating Optical Fiber; U.S. Pat. No. 5,732,178 to Y. Terasawa et al. entitled Single Mode Optical Fiber; U.S. Pat. No. 5,742,723 to M Onishi et al. entitled Optical Transmission System With Dispersion Compensating Optical Fiber; U.S. Pat. No. 5,748,824 to D. Smith entitled Positive Dispersion Optical Waveguide; U.S. Pat. No. 5,761,366 to S. Oh et al. entitled Optical Fiber With Smooth Core Refractive Index Profile And Method of Fabrication; and U.S. Pat. No. 5,822,488 to Y. Terasawa et al. entitled Single-mode Optical Fiber With Plural Core Portions, incorporated herein by reference.